Abstract

Abstract
The assemblage Zn-poor hercynite + quartz, occurring with cordierite, garnet and/or sillimanite, is diagnostic of high-temperature granulite-facies metamorphism at mid-crustal depths. It has been recorded from a handful of terrains worldwide, in which peak temperatures probably exceeded 800°C, and pressures were typically in the range 4 - 7 kbar, lower than that required for the association sapphirine + quartz. A typical spinel composition is (Fe2+0.75Mg0.25)(Al1.9Fe3+0.1)O4, and hercynite generally has a lower XMg than coexisting garnet. Osumilite may occur with hercynite, or in associated rocks of higher bulk XMg. Uncertainty in the absolute P-T conditions of hercynite-quartz rocks results from unresolved inconsistencies in experimental studies of cordierite equilibria, and from the difficulty of reconstructing the peak compositions of minerals, particularly of oxides, which generally show exsolution and corona textures. The coronas record information about the retrograde P-T path, while in some cases larger-scale mineral associations, such as hercynite-quartz aggregates replacing cordierite porphyroblasts. reveal part of the prograde history. Most hercynite-quartz terrains lack evidence for an earlier high-pressure history, and cooled isobarically from a very hot, transient geotherm. The most generally applicable models involve a combination of magmatic advection and lithospheric thinning. Some terrains show a striking anticlockwise P-T path, for which the best explanation is that voluminous crustally-derived magmas were transferred to higher levels, moving past and displacing downwards the supracrustal rocks from which the P-T path is deduced.